Microstructured reconfigurable composite material

ABSTRACT

A microstructured reconfigurable or morphing composite material with controlled anisotropic deformation properties. The composite material provides highly controlled deformation and stiffness properties. Microscopic three dimensional structures are included in the composite material to control its deformation kinematics and stiffness properties. The composite material has highly segregated in-plane and out-of-plane stiffness properties.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/212,469, filed on Sep. 17, 2008, issued as U.S. Pat. No.8,409,691 on Apr. 2, 2013, which claims priority to and the benefit ofU.S. Provisional Application No. 60/973,004, filed on Sep. 17, 2007. Theentire content of the applications set forth above are incorporatedherein by reference. This application is also related to U.S. patentapplication Ser. No. 11/193,148, filed on Jul. 29, 2005, issued as U.S.Pat. No. 7,550,189 on Jun. 23, 2009, the entire content of which isincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with U.S. Government support under Contract No.FA8650-06-C-5059 awarded by the Air Force Research Laboratory. The U.S.Government has certain rights to this invention.

BACKGROUND

1. Field of the Invention

The present invention is related to composite materials, and moreparticularly, to reconfigurable composite materials.

2. Description of Related Art

Conventional structural design typically uses fixed geometry structuralelements designed to provide acceptable performance over a range ofoperating conditions. However, fixed geometry structural elements limitthe degree of optimization that can be achieved over the range ofoperating conditions. Reconfigurable surface technology, which permitsstructural components to undergo large-scale, in-service changes incomponent geometry could provide performance enhancements of thestructural design over broad operating conditions. Additionally,reconfigurable surface technology could enable a structural design withmulti-function capabilities, optimized wave interactions (e.g.,electromagnetic, shock, sound, air flow), and improveddeployment/storage usability for traveling among others.

Applications that may benefit from reconfigurable surface technologyinclude aircraft wings, control surfaces, and field-deployablestructures. Reconfigurable (or morphing) surfaces require largereversible deformations, low parasitic mass, reconfiguration speedappropriate to the application, high degree of shape control and abilityto scale to large areas. Morphing or reconfigurable structurespotentially allow for previously unattainable performance by permittingseveral optimized structures to be achieved using a single platform (orstructure). New engineered materials (e.g., a composite material) thatmay achieve the necessary deformations but limit losses in parasiticactuation mass and structural efficiency (stiffness/weight) are needed.These new materials should exhibit precise control of deformationproperties and provide high stiffness when exercised through largedeformations.

Prior approaches to achieve a reconfigurable or morphing structure suchas a morphing airplane wing rely on designs that are relativelymass-inefficient as compared to conventional airframe design. Theinefficiency results from the need to use soft, flexible materials suchas shape memory polymer matrix composites as the wing skin, forcingstructural mass to be concentrated in the interior where it reducesstiffness of the whole structure, as compared to torsion box designs.

Shape memory polymer matrix composites have shown reversible deformationrecovery through the combined effect of the energy stored in thereinforcement phase and from the shape memory effect of the polymer.Prior approaches utilize composite fiber reinforcement such as carbonfiber in uni-axial, cross-ply, or woven configurations. The principalchallenges in using fibers for these applications are theinextensibility of the fibers, limiting the deformation primarily tobending, and the poor stability of the fiber in compression leading tomicrobuckling.

Therefore, there is currently much interest in reconfigurable ormorphing structures capable of performing large changes in variousphysical configurations (e.g., a wing or engine inlet) such thatoptimized performance may be achieved over a broad range of operationalconditions. It is desirable to have materials that provide controllablestiffness properties and large deformation. It is also desirable to havematerials with attributes suitable for morphing structures such asrelatively low in-plane axial stiffness for efficient shape changing,combined with good resistance to out of plane deformations such as thoseexerted by pressure loading of air or water.

SUMMARY OF THE INVENTION

Aspects of embodiments of the present invention are directed toward amicrostructured reconfigurable or morphing composite material withcontrolled anisotropic deformation properties. The microstructuredreconfigurable composite material provides highly controlled deformationand stiffness properties. In some embodiments of the present invention,microscopic three dimensional structures are included in the compositematerial to control its deformation kinematics and stiffness properties.In some embodiments of the present invention, the composite material hashighly segregated in-plane and out-of-plane stiffness properties.

According to an embodiment of the present invention, a microstructuredcomposite material with controlled deformation is provided. Themicrostructured composite material includes a matrix material and atleast three reinforcement layers embedded in the matrix material. The atleast three reinforcement layers are embedded in the matrix material,and each of the at least three reinforcement layers includes a pluralityof platelets arranged along a plane of the matrix material. A pluralityof connecting members interconnect the plurality of platelets along theplane of the matrix material. Means is provided for mechanicallycoupling stress between adjacent layers of the at least threereinforcement layers to mitigate out-of-plane deformation of the matrixmaterial while allowing deformation of the matrix material along theplane.

The microstructured composite material may further include areinforcement member on at least one of the at least three reinforcementlayers. The reinforcement member has an elongated shape and extends in adirection substantially parallel to the at least one of the at leastthree reinforcement layers.

The plurality of platelets may be arranged in a cellular pattern. Theplurality of platelets may be arranged in a lattice pattern.

Each of the plurality of connecting members may have a straight section.Each of the plurality of connecting members may have a curved section.

The matrix material may be a variable elastic modulus material. Thevariable elastic modulus material may be selected from the groupconsisting of shape memory polymer, shape memory alloy, phase changingmetal, wax, ice, plastically deforming material, electrorheologicalfluid, magnetorheological fluid, electrostrictive material,piezoelectric material, magnetostrictive material, ferromagneticmagnetostrictive material, magnetorheological elastomer,electrorheological elastomer, and liquid crystal elastomer.

The matrix material may be a constant elastic modulus material.

According to an embodiment of the present invention, a microstructuredcomposite material with controlled deformation includes a matrixmaterial and at least two reinforcement layers embedded in the matrixmaterial. Each of the at least two reinforcement layers includes aplurality of support members. The support members of each layer of theat least two reinforcement layers extend substantially in parallel alonga plane of the matrix material. The support members of a layer of the atleast two reinforcement layers cross the support members of an adjacentlayer of the at least two reinforcement layers. A plurality ofconnecting members interconnect the support members of adjacent layersof the at least two reinforcement layers at crossings between thesupport members. Two support members of the plurality of support membersare pivotably interconnected by a corresponding one of the plurality ofconnecting members to mitigate translation between the two supportmembers while allowing rotations between the two support members alongthe plane.

The plurality of connecting members may be pins.

The at least two reinforcement layers may include at least threereinforcement layers, and the support members of alternate layers of theat least three reinforcement layers extend in the same direction.

The support members of a layer of the alternate layers may be offsetfrom those of another layer of the alternate layers in a direction alongthe plane.

According to an embodiment of the present invention, a microstructuredcomposite material with controlled deformation includes a matrixmaterial and at least two reinforcement layers embedded in the matrixmaterial. Each of the at least two reinforcement layers includes atleast two platelets arranged along a plane of the matrix material. Afirst connecting member extends along the plane and interconnects two ofthe at least two platelets of a first layer of the at least tworeinforcement layers. A second connecting member extends along the planeand interconnects two of the at least two platelets of a second layer ofthe at least two reinforcement layers. The second layer is adjacent tothe first layer. The second connecting member crosses and interlockswith the first connecting member to mitigate translation between thefirst connecting member and the second connecting member while allowingrotation between the first connecting member and the second connectingmember along the plane.

According to an embodiment of the present invention, a microstructuredcomposite material with controlled deformation includes a matrixmaterial and at least one reinforcement layer embedded in the matrixmaterial. The at least one reinforcement layer includes a plurality ofsupport members arranged in a pattern along a plane of the matrixmaterial. Each of the plurality of support members includes a firstsupport member, a second support member and a third support member. Thefirst support member has a protrusion extending from a surface of thefirst support member and a first edge. The second support member facesthe first support member and is structured to receive the protrusion.The second support member has a second edge. The third support member ispivotably connected between the first support member and the secondsupport member, and has a third edge and an opening. The protrusiontraverses the opening. The third support member is configured to beblocked from pivoting around the protrusion in at least one directionwhen the third edge is in contact with the first edge and the secondedge.

The first support member may include a fourth edge, the second supportmember may include a fifth edge, the third support member may include asixth edge. The third support member may be configured to be blockedfrom pivoting around the protrusion in a direction different from the atleast one direction when the sixth edge is in contact with the fourthedge and the fifth edge.

The protrusion may have a plate shape, and the second support member mayhave a slot for receiving the protrusion.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, together with the specification, illustrateexemplary embodiments of the present invention, and, together with thedescription, serve to explain the principles of the embodiments of thepresent invention.

FIG. 1 a is a drawing illustrating a schematic perspective view of acomposite material. including a reinforcement layer according to anembodiment of the present invention.

FIG. 1 b is a drawing illustrating a plan view of a design of areinforcement layer of a composite material according to an embodimentof the present invention.

FIG. 1 c is a drawing illustrating a cross sectional view of one of theligaments shown in FIG. 1 b.

FIG. 2 is a simplified view of a section of the reinforcement layershown in FIG. 1 b.

FIG. 3 is a drawing illustrating a plan view of a reinforcement layeraccording to an embodiment of the present invention.

FIG. 4 is a cross sectional view of a composite material according to anembodiment of the present invention.

FIGS. 5 a, 5 b and 5 c are drawings showing cross sectional views of acomposite material having through thickness mechanical connectionsaccording to an embodiment of the present invention.

FIGS. 6 a, 6 b and 6 c are drawings showing cross sectional views of acomposite material having non-connected platelets.

FIGS. 7 a and 7 b are drawings illustrating the strain distributionthrough a beam or plate member subject to bending and axial stretching.

FIGS. 8 a and 8 b are drawings in perspective views illustratingreinforcement layers of a composite material according to an embodimentof the present invention incorporating interlocking joints betweenplatelets.

FIG. 8 c is a drawing illustrating a cross sectional view of areinforcement layer illustrated in FIGS. 8 a and 8 b.

FIGS. 9 a and 9 b are drawings in plan view and cross sectional view,respectively, illustrating through thickness connected reinforcementlayers according to an embodiment of the present invention.

FIG. 9 c is a drawing illustrating two exemplary reinforcement elementsaccording to embodiments of the present invention.

FIGS. 10 a and 10 b are drawings in plan view and cross sectional view,respectively illustrating through thickness connected reinforcementlayers according to another embodiment of the present invention.

FIG. 10 c is a drawing illustrating a cross sectional view of areinforcement element shown in FIGS. 10 a and 10 b.

FIGS. 11 a, 11 b, 11 c, 11 d and 11 e are drawings illustrating areinforcement layer according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Detailed descriptions will be made below in reference to certainexemplary embodiments according to the present invention. The drawingsand descriptions are to be regarded as illustrative in nature and notrestrictive.

Embodiments of the present invention provide reconfigurable (ormorphing) composite materials that provide highly controlled deformationand stiffness properties. The construction and operation of thesematerials with machine like properties macroscopically are enabled byutilizing machine like microstructures included in the material.Microscopic three dimensional (3D) structures are included in thematerials to control their deformation and stiffness properties.Exemplary microscopic 3D features that may be included into thecomposite materials include bearing, ball and pin joints, and detents.The reconfigurable composite materials according to the embodiments ofthe present invention allow a range of new material properties to beenabled. These include highly segregated in-plane and out-of-planestiffness properties that are highly desirable, for example, in morphingaircraft structure applications. Other desirable features includecontrollable non-linear stiffness properties for creating controllablesoft and stiff properties and deformation locking.

Furthermore, the embodiments of the present invention improve onexisting non-tailored materials for morphing structures such as variablestiffness composites using fibers or flat plate reinforcements byproviding additional design freedoms to incorporate non-linear materialproperties. For example, the embodiments of the present inventionincorporate machine-like elements at the microstructure level tocarefully tailor the material properties to application requirements.This allows for a much improved control of material properties ascompared to conventional composite materials. Furthermore, certain typesof anisotropy in material properties such as in-plane and out-of-planesegregation that can be realized through the embodiments can not beachieved with conventional composite materials technology due to thelack of control over shape and mechanical coupling of reinforcements.

In reconfigurable composite material applications, there is often a needto allow large deformations and relatively small stiffness in onedirection or orientation while permitting small deformations in anotherorientation. An example is the construction of morphing aircraft wingswhere the wings are designed to be stretched in a direction parallel tothe plane of the wing, but it is required that the membrane deflectiondue to air pressure loading be minimized. The exemplary embodiments ofthe present invention described below provide some configurations thatmay be applied to construct morphing aircraft wings, however, thepresent invention is not limited thereto.

FIG. 1 a is a drawing illustrating a perspective view of a compositematerial 100 according to an embodiment of the present invention. Thecomposite material 100 includes a plurality of reinforcement layers 102embedded in a matrix material 104. In some embodiments, thereinforcement layers 102 include at least three layers.

FIG. 1 b, according to an embodiment of the present invention,illustrates a design for a layer of the reinforcement layers 102 thatcan be used to construct a reconfigurable composite material designedfor discriminating in-plane loading from out-of-plane loading. In thecomposite material 100, several reinforcement layers 102 may be stackedon top of each other with suitable offsets in the in-plane directions toachieve desirable stiffness properties. The reinforcement layers 102 areconnected through their thickness by the matrix material 104 that allowsfor shear stress transfer between the reinforcement layers 102. Each ofthe reinforcement layers 102 has been optimized for shear loading. Insome cases, this may be accomplished using a shear frame. For the easeof description, a simplified representation of a shearing frame 112 isshown in FIG. 2, where the boundaries 114 of the shearing frame 112 arekept at constant length, and the angles 116 between the frame members118 are altered to achieve a change in shape and area of the shearingframe 112.

In FIG. 1 b, each of the reinforcement layers 102 includes a network ofplatelets 120 as support members and ligaments 122 for connecting theplatelets 120. The design of the platelets 120 and the ligaments 122 mayfollow from design of cellular materials and compliant mechanisms. See,Gibson, L., Ashby, M., Cellular Solids: Structure & Properties, PergamonPress, New York, 1988; and Howell, Larry, Compliant Mechanisms, JohnWiley and Sons, New York, 2001; the entire contents of both referencesare incorporated by reference herein.

FIG. 1 c is a drawing illustrating a cross sectional view of one of theligaments shown in FIG. 1 b.

Both the platelets 120 and the ligaments 122 may be of arbitrary shape,and different designs provide for different stiffness and straincapacity. The ligaments 122 should provide high stiffness in tension,but may bend easily and reversibly to reduce the likelihood that thecomposite material 100 may undergo plastic or irreversible deformation.Referring to FIG. 1 c, it shows a cross sectional view of one of theligaments 122 across the line A-A′. In some embodiments, the ligaments122 may have a high aspect ratio beam structure where the aspect ratiois defined as the thickness in the width direction (W) divided by thedepth direction (D) as shown in FIG. 1 c. In some embodiments accordingto the present invention, the aspect ratio may be at least 1 (i.e., asquare cross section) and may be as high as 4-10. Using isotropicetching processes and metals such as steel, aluminum or copper; aspectratio approximately 2.5:1 may be achieved that allows a great deal offlexibility of the ligaments 122 using in-plane bending. Othercompatible processes such as laser cutting or water jet machining mayprovide larger aspect ratios. The shape of the ligaments 122 may also bea design variable. FIG. 3 shows an exemplary reinforcement layer 102′according to an embodiment of the present invention with curvedligaments 122′. A straight ligament may be bent into a curve ligamentusing external forces and locally compressive strains. The initiallycurved ligament 122′ allows straightening in response to external loadsand may respond flexibly to either tension or shear loads.

Referring back to FIGS. 1 a and 1 b, the design of the platelets 120 isa balance between the needs of deformation and stiffness of thecomposite material 100. The higher the deformation, the more open matrixspace should be included in the composite material 100, while stiffnessmay be increased by volume fraction. The shape of the platelets 120 andthe arrangement of the platelets 120 (e.g., a cellular or latticepattern) are driven by the type of deformation that is necessary andparticularly the Poisson ratio of deformation needed. A compositematerial constructed with multiple layers of the exemplary reinforcementlayer 102 shown in FIG. 1 b is effective for shearing loads. Other typesof loads such as compression or tension with Poisson ratio ranging from−0.5 to >1 may be accommodated using various types of arrangements ofthe platelets and ligaments. The design of such material can be based oncellular materials design in which a variety of Poisson ratios have beendemonstrated by altering the basic cell dimensions. See, L. Gibson andM. Ashby. In general, one design consideration is to maximize the areafraction of the reinforcement platelets 120 to achieve increasedstiffness and strength while still permitting the desired deformation ofthe composite material 100. For example, the use of triangular,rectangular, hexagonal symmetries may allow for a wide variety ofmechanical properties and angular dependencies to be achieved.

In some embodiments of the present invention, the ligaments 122 may alsobe made for a non-load bearing function that serves to maintain spacingand separation of the platelets 120 during fabrication. In theseembodiments, it may be desired that the platelets 120 be connectedphysically with little to no force once the composite material isfabricated. The ligaments 122 should be sturdy enough, however, tomaintain the shape and layout of the platelets 120 during fabricationsteps. Exemplary designs of the ligaments 122′ are curved members (shownin FIG. 3) with relatively high aspect ratio such that the bendingstiffness of the ligament 122′ itself is low, and such that theeffective physical axial connectivity of the platelets 120 is weak. Thisallows the platelets 120 to behave independently once the compositematerial 100 is fabricated. FIG. 3 shows one non-limiting example ofthis type of ligament design to connect an array of hexagonal platelets120′. Many other designs may provide similar functionality includingstraight sections with nodes (shape of V or W), serpentine shapes, ringtype springs, and others. Another benefit of this approach is that itallows the platelets 120 to be electrically connected through theligaments 122 while physical coupling among the platelets 120 are mainlyprovided through the shear stress of the matrix material. This benefitmay be useful in applications where it is desirable to maintain anelectrical connectivity through out the composite material such as isnecessary for protection from lightning strike or for purposes ofreflection of electromagnetic waves.

FIG. 4 shows a cross sectional view of a composite material 100′ withlong thin reinforcement members 130 incorporated over the ligaments 122connecting the platelets 120 in multiple reinforcement platelet layers132 according to another embodiment of the present invention. The longreinforcement members 130 that span the frame 112 and have orientationparallel to one set of the frame members 118 may be included in theembodiment. The reinforcement platelet layers 132 and the reinforcementmembers 130 (e.g., a long thin cable) are alternated through thethickness of the composite material 100′. The matrix material 104, e.g.,a shape memory polymer (SMP), may surround all of the reinforcementelements including the platelets 120, the ligaments 122 and thereinforcement members 130.

The matrix material 104 may be a non-variable elastic modulus material(i.e., always flexible with a constant elastic modulus) or variableelastic modulus material. Suitable non-limiting exemplary non-variableelastic modulus material includes members of the elastomer family ofamorphous polymers used above their glass transition temperature.Examples include Natural Rubber (NR), Synthetic Polyisoprene (IR), Butylrubber (copolymer of isobutylene and isoprene, IIR), Halogenated butylrubbers (Chloro-butyl Rubber: CIIR; Bromobutyl Rubber: BIIR),Polybutadiene (BR), Styrene-butadiene Rubber (copolymer of polystyreneand polybutadiene, SBR), Nitrile Rubber (copolymer of polybutadiene andacrylonitrile, NBR), also called Buna N rubbers, Hydrogenated NitrileRubbers (HNBR) Therban and Zetpol, Chloroprene Rubber (CR),polychloroprene, Neoprene, Baypren, EPM (ethylene propylene rubber, acopolymer of ethylene and propylene) and EPDM rubber (ethylene propylenediene rubber, a terpolymer of ethylene, propylene and adiene-component), Epichlorohydrin rubber (ECO), Polyacrylic rubber (ACM,ABR), Silicone rubber (SI, Q, VMQ), Fluorosilicone Rubber (FVMQ),Fluoroelastomers (FKM, and FEPM) Viton, Tecnoflon, Fluorel, Aflas andDai-El, Perfluoroelastomers (FFKM), Tecnoflon PFR, Kalrez, Chemraz,Perlast, Polyether Block Amides (PEBA), Chlorosulfonated Polyethylene(CSM), (Hypalon), Ethylene-vinyl acetate (EVA), Thermoplastic elastomers(TPE), and Thermoplastic Vulmayizates (TPV), for example Santoprene TPV,Thermoplastic Polyurethane (TPU), Thermoplastic Olefins (TPO).

Exemplary suitable variable stiffness materials for the matrix material104 include shape memory polymers, shape memory alloys, phase changingmetals, wax, ice, plastically deforming materials, electrorheologicalfluids, magnetorheological fluids, electrostrictive materials,piezoelectric materials, magnetostrictive materials, ferromagneticmagnetostrictive materials, magnetorheological elastomers,electrorheological elastomers, and liquid crystal elastomers. Each ofthese potential materials brings a different set of properties thatinfluence the ultimate functionality of the composite material. Forexample, some materials such as shape memory polymers and wax providevery high strain capacity to enable large deformations in the material,while others provide small deformations but very rapid stiffness tuningsuch as piezoelectric and magnetostrictive materials.

In another embodiment according to the present invention, the resistanceto out-of-plane deformation of thin plate members subject toout-of-plane pressures may be increased by the use of through thicknessmechanical connections such as pins and holes or slots. Suitable holesor slots are formed on adjacent overlapping thin plate members which arecoupled together by receiving the pins in the thickness direction. Thatis, these pins mechanically couple the adjacent overlapping layers.FIGS. 5 a-5 c show the concept for the through thickness mechanicalconnections in a rectangular platelet reinforcement composite material140. The through thickness connectors (e.g., pins) 142 in FIGS. 5 a-5 cmay be circular pins that are mated to circular sockets in thevertically aligned platelets 144. The connectors 142 may be of othershapes including, for example, rectangular or triangular in order toalter the relative bending and twist resistance of the stacks ofplatelets 144 connected by the connectors 142. The structure shown inFIGS. 5 a-5 c limits the non-symmetric displacement of the platelets 144through the thickness of the material as is typically encountered inbending or membrane deformation.

By providing through thickness linking between the platelets 144,antisymmetric deformation where the platelets 144 must translate orrotate in opposite directions through the thickness of the material islocked out. However, the platelets 144 are relatively free to displacetogether to accommodate stretching relatively easily because theconnectors 142 will not significantly affect the in-plane properties ofthe reinforcement composite material 140.

In practice, one must consider the strain distribution through a beam orplate member subject to bending and axial stretching as illustrated inFIGS. 7 a and 7 b, respectively. In the case of bending (e.g., membranebulging) shown in FIG. 7 a, the strain is anti-symmetric about theneutral axis (e.g., X-axis in FIG. 7 a) of the material. In the case ofpurely axial loading shown in FIG. 7 b, strain is generally symmetricabout the midplane of the material, and therefore symmetric about theneutral axis (e.g., X-axis in FIG. 7 b). If one aspires to prescribedifference in stiffness of a material in response to bending and axialloads, such as in morphing applications, it is necessary to control thekinematics of the material to respond differently to symmetric andantisymmetric strain loads.

Referring back to FIGS. 5 a-5 c, therefore, by linking the platelets 144in the composite material 140 through the thickness, a difference in thelocal stiffness is achieved by changing the deformation kinematics ofthe composite material 140. FIGS. 5 a-5 c and FIGS. 6 a-6 c illustratethe differences in behavior for the linked and non-linked plate systemsfor bending and stretching. As shown, stretching behavior is similar asshown in FIGS. 5 b and 6 b, but bending behavior is much different asshown in FIGS. 5 c and 6 c. As shown in FIG. 6 c, the non-connectedplatelets 144′ may accommodate bending by moving together on thecompressive side and apart on the tensile side. However, the connectedplatelets 144 in FIG. 5 c may only rotate to accommodate the bendingmotion, which is a much higher energy deformation process due to therelative incompressibility of the matrix material 146. By providingthrough thickness linking, the antisymmetric deformation mode may belocked out and the platelets 144 translate or rotate in oppositedirections through the thickness of the composite material 140. However,the platelets 144 are free to displace in unison to accommodatestretching relatively easily.

In FIGS. 5 a-5 c, the matrix material 146 of the composite material 140plays an important role. In conventional platelet composites such as theexample shown in FIGS. 6 a-6 c, the composite material 140′ gains itsstiffness and strength through the transfer of load from onereinforcement member (e.g., platelets 144′) to another through shear ofthe matrix material 146′. The stiffness of the matrix material 146′ willdetermine the compliance of this connection and to some extent theoverall stiffness of the composite material 140′. Therefore, theconstruction described in reference to FIGS. 5 a-5 c may benefit fromthe use of variable stiffness matrix materials, such as shape memorypolymers, phase transition materials such as wax and low meltingtemperature metals, where a high stiffness matrix state provides goodmechanical rigidity, and a low stiffness state provides good strain anddeformation capacity necessary for shape changing operations likemorphing. Alternatively, due to the discrimination of in-plane andout-of-plane properties (e.g., stiffness), the composite material 140may include low stiffness matrix materials such as elastomers (e.g.,silicone, EPDM, urethane, etc) as its matrix material 146. These matrixmaterials allow large shear strains, and therefore would provide forlarge deformation strain suitable for structural shape changingapplications.

The embodiment shown in FIGS. 5 a-5 c limits the rotation of theplatelet stacks 148, as would be experienced during bending or membranetype deformations. The connectors (e.g., pins) 142 do not significantlyaffect the in-plane properties of the composite material 140. Thereforethe ratio of in-plane stiffness to out-of-plane (e.g., bending)stiffness may be significantly increased.

The construction shown in FIGS. 5 a-5 c may be achieved using metallicreinforcement layers such as aluminum, steel, titanium and the like.These metallic layers may be processed using photolithographic etchingtechniques such as differential or timed etching. Mechanical processessuch as punch and stamping may also provide similar types of features inmetal sheets. Other methods that may be suitable are positive depositionmethods such as laser sintering. The design of the connectors 142 (e.g.,pins, posts) including length, size, aspect ratio is tailorable to someextent using suitable manufacturing techniques. The connectors 142should provide connectivity between the platelets 144 and restrict theirmotion laterally, and therefore should be sufficiently stiff and strongto manage this function. In some embodiments, the connectors 142 may behollow (e.g., a tube).

FIGS. 8 a and 8 b show fundamental unit cells of reinforcement layers ofa composite material according to an embodiment of the present inventionincorporating interlocking joints between platelets in two differentperspective views. FIG. 8 c shows a cross sectional view of the unitcells shown in FIGS. 8 a and 8 b along the line B-B′.

In the embodiment shown in FIGS. 8 a and 8 b, ligaments 150 (orconnecting members) with a suitably high aspect ratio connect platelets152 in the thickness direction. The direction T in FIG. 8 c indicatesthe thickness direction. This type of connection will allow theplatelets 152 to rotate in direction along the plane but not translatein response to bending loads. The rotation is enabled, for example, bythe bending of the ligaments 150 about the intersection point, andtherefore, the amount of rotation allowed is determined by the aspectratio (e.g., slenderness) of the ligaments 150. In the embodiment shownin FIGS. 8 a and 8 b, successive layers of reinforcement platelets 152are joined through the thickness using overlapping joints 154. Theseoverlapping joints 154 may be created using etching techniques or viastamping and punch techniques. The overlapping joints 154 couple thetranslation of the platelets 152 while allowing rotation of theplatelets 152 in successive layers. The embodiment may be useful ingenerating a shear type loading while increase the out-of-planestiffness because membrane deflections require that layers through thethickness direction translate with respect to one another.

FIGS. 9 a and 9 b are drawings in plan view and cross sectional view,respectively, illustrating through thickness connected reinforcementlayers according to an embodiment of the present invention.

FIGS. 9 a and 9 b illustrate an embodiment of through thicknessconnected reinforcement layers of a composite material that have highshear deformation capacity, combined with high resistance toout-of-plane bending loads. The embodiment shown in FIGS. 9 a and 9 butilizes a geometry with high aspect ratio intersecting members that arepinned into each other to provide essentially one degree of freedom formorphing and shape changing operations. By connecting the members with,for example, pin joints, a much different local kinematics is achievedthan is possible with fiber composite layers. This is because of theconstraining effect of the pin joints on the kinematics of thereinforcement system. It should be appreciated that these pin joints aresmall in the sense that there are many joints per unit thickness of thecomposite material. This scale discriminates the composite material frompreviously established technologies at the macro scale such as latticenetworks and the like.

FIG. 9 a shows two reinforcement layers according to an embodiment ofthe present invention. FIG. 9 b shows a cross sectional view showingfive reinforcement layers according to an embodiment of the presentinvention. Each of the reinforcement layers shown in FIGS. 9 a and 9 bincludes a plurality of reinforcement elements 201 extending inparallel. In FIG. 9 a, the reinforcement elements 201 of the top layer200 of the two reinforcement layers extend in a horizontal direction.The reinforcement elements 201 of the bottom layer of the tworeinforcement layers extend in a vertical direction. As such, theembodiment shown in FIGS. 9 a and 9 b includes a lattice ofreinforcement elements 201 (or support members) where nodes 202 atcrossings of the reinforcement elements 201 are connected by, forexample, pin connectors that allow rotation of the reinforcementelements 201 relatively freely, but restrict translation of thereinforcement elements 201. The embodiment of FIGS. 9 a and 9 b permitshearing deformation but restricts deformation in other dimensions.Although FIGS. 9 a and 9 b show only the lattice of reinforcementelements 201, the reinforcement elements 201 will be surrounded by amatrix material that exhibits a phase change such as a shape memorypolymer, wax, or elastic materials (e.g., rubbers and silicones). Thisprovides the lateral stiffness and locks the kinematics mechanism intoplace. However, one skilled in the art would appreciate that othersuitable types of matrix materials may be applied.

Referring to FIG. 9 a, in practice, multiple reinforcement layersoverlapping each other and laminated through the thickness may be used.FIG. 9 b is a cross sectional view of the embodiment shown in FIG. 9 aalong the line A-A′ showing multiple layers of overlapping reinforcementlayers. By increasing the number of reinforcement layers, the thicknessof a composite material may be increased and additional load can besupported. Furthermore, the bending stiffness in this embodiment can beincreased if the reinforcement layers are all coupled through thethickness. Furthermore, the reinforcement elements 201 may benon-uniform in shape along their length direction such as wavy or sawtooth patterns to further adjust the local strain generated in thematrix phase of a composite material including the reinforcement layersduring deformations. In some embodiments, at least three overlappingreinforcement layers are provided.

FIG. 9 c is a drawing illustrating two exemplary reinforcement elementswith wavy and saw tooth shapes according to other embodiments of thepresent invention.

FIGS. 10 a and 10 b illustrate an embodiment of a composite materialwith multiple reinforcement layers, each layer including a plurality ofreinforcement elements 304 extending in parallel, the reinforcementlayers staggered through the thickness to achieve a less permeable,higher and more uniform stiffness behavior. For example, the topreinforcement layer 300 includes reinforcement elements 304 extending ina horizontal direction. This embodiment may be employed in aerostructures, for example, where a pressure is loaded onto the surface ofa composite material including the reinforcement layers. In order tostagger the reinforcement layers, pins 302 may be located at differentlocations in successive layers. In some embodiments, each of thereinforcement layers acts independently as a shearing frame, but thereinforcement layers also are coupled through the shear stress generatedin a matrix material embedding or enclosing the reinforcement layersduring loading as in conventional fiber and platelet compositematerials. In other embodiments, there is not a pin 302 at everycrossing to reduce the number of pins 302 per crossing. Reducing thenumber of pins 302 may increase the flexibility of the compositematerial and allow it to accommodate more strain and the effects ofnon-ideal boundary conditions. The flexibility of the reinforcementelements 304 may also be adjusted by utilizing architecturally compliantshapes such as wavy strips or textured strips.

Additional variations of the size and design of the shapes of thereinforcement elements 304 include repeating hourglass shapes, plateletswith connecting ligaments, discreet platelets, etc. As mentioned above,certain materials may increase performance of the reinforcement layerswhen the reinforcement members 304 have mechanical complianceincorporated, for example, as in corrugations. Also, the aspect ratio ofthe reinforcement elements 304 may be altered to be either wider orthinner as required. FIG. 10 c is a drawing illustrating a crosssectional view of a reinforcement element shown in FIGS. 10 a and 10 b.The aspect ratio of the reinforcement element 304 is defined as theratio of the width (W) to the depth (D) of the cross section of thereinforcement element 304 as shown in FIG. 10 c. In addition,overlapping of the reinforcement layers provide a sliding plate actionduring the shearing of a composite material incorporating thereinforcement layers. This may be used to make the composite materialimpermeable and suitable for operation such as a morphing wing skin orother components with pressure loading during large deformations.

FIGS. 11 a, 11 b, 11 c, 11 d and 11 e are drawings illustrating areinforcement layer according to an embodiment of the present invention.

FIGS. 11 a-11 c illustrate a reinforcement layer 400 of a compositematerial in plan view and cross sectional view along the lines A-A′ andB-B′, respectively, according to an embodiment of the present invention.FIG. 11 e is an exploded perspective view of a section of thereinforcement layer 400. For the ease of illustration, a matrix materialthat may embed or enclose the reinforcement layer 400 is not shown inFIGS. 11 a-11 e. This embodiment is another variation on the pinnedrotating shearing reinforcement scheme shown in the embodiments of FIGS.9 a, 9 b, 10 a and 10 b. The height of reinforcement members 402 arechanged in their thickness direction to create locking structures. Onlya section of the reinforcement layer 400 is shown in FIGS. 11 a-11 e.One skilled in the art would appreciate that the reinforcement layer 400may include many repeated units of the structure shown in FIGS. 11 a-11e. As shown in FIGS. 11 a-11 e, two separate reinforcement members 402are pinned together via a pin 405 with a third reinforcement member 403in between such that there is an overlap region at their edges. In theembodiment shown in FIGS. 11 a-11 e, the pin 405 may be formed on one ofthe two reinforcement members 402, and an opening or socket formed onthe other one of the reinforcement members 402 is suitably sized toreceive the pin 405 to securely couple the two reinforcement members 402together. The pin 405 goes through an opening 407 (shown in FIG. 11 e)formed on the reinforcement member 403, the opening 407 being suitablysized to allow the pin 405 to rotate freely. However, one skilled in theart would appreciate that the two reinforcement members 402 may becoupled together by other suitable methods.

When the reinforcement members 402 are rotated in relation to the thirdreinforcement member 403, for example when undergoing shear deformation,the reinforcement members 402 are free to turn or rotate until theiredges engage the edges of the corresponding third reinforcement member403 as shown in FIG. 11 d. At this point, the stiffness of a compositematerial incorporating the reinforcement layer 400 increasessignificantly. The same effect may be generated for the reversedirection as well. This effect provides an effective hard stop againstfurther deformation, thereby providing high stiffness for the compositematerial incorporating the reinforcement layer 400 in one or another ofthe end configurations. This same architecture may be used in tension orcompression (rather than pure shear) by rotating the axis of thereinforcement members 402 and the third reinforcement member 403 by 45°.

The above described embodiments may be prepared using standardphotolithography processes commonly used in the printed circuit boardand metal etching industries. For example, metal layers such asaluminum, steel, titanium, copper, etc., are patterned and etched usingprinted circuit board type photolithography. In this process, a mask iscreated and used to pattern a photoresist coating previously applied tothe metal. The photoresist and metal are then subject to etchingprocessing, for example, using acid for material removal, wherebyfeatures may be created in the metal layers. Once the metal layers arefabricated they may be incorporated with a shape memory polymer or othervariable stiffness matrix material. Current processes allow thesefeatures to be created down to micron levels and across large areas ofat least square meters. Therefore, the various architectures shown inthe embodiments are compatible with processes that allow forintroduction of these composite materials as structural compositematerials for large scale applications such as aircraft and othervehicles.

By way of an example, a thermoplastic shape memory polymer (e.g., MHIDiaplex 55° C. Tg polyurethane SMP) and metallic layers are interwoven,and the assembly is consolidated under heat and pressure. In otherembodiments, thermoset shape memory polymers such as Conerstone ResearchGroup Veriflex material may be used to manufacture the laminatematerial. In these embodiments, metallic layers are held together andseparated using spacer materials. For alignment purposes, one may use apeg and hole type system to ensure that all layers are laterallyregistered with respect to one another. This stack of layers is thenplaced into a mold and infiltrated with an uncured resin. By curing theresin, the microstructure may be fabricated. In fabricating 3Dstructures such as pegs and holes, lateral alignment is necessary toensure that the features interlock as desired through the thickness ofthe material.

Various features may be fabricated using the above described techniques.For example, through holes may be fabricated using traditionalphotolithography. Pegs or pins such as those required in FIGS. 9 a-9 b,10 a-10 b and 11 a-11 e may be made using timed, differential etchingtechniques, whereby the etchant's rate of etching is combined withspecialized masking techniques to make posts or pins and other elevationrises from thicker sheets of metal.

Features such as those shown in FIGS. 8 a-8 b may be fabricated byutilizing the above discussed etching process to create a planestructure, which is then formed into 3D structures using stamping andbending processes. The 3D structures may potentially be fabricateddirectly using differential etching techniques. The plane units are thenaligned and stacked to create the 3D structures. The plane units may beheld together by mechanical interlocking, adhesive bonding, or brazingand soldering.

While the present invention has been described in connection withcertain exemplary embodiments, it is to be understood that the inventionis not limited to the disclosed embodiments, but, on the contrary, isintended to cover various modifications and equivalent arrangementsincluded within the spirit and scope of the appended claims, andequivalents thereof.

What is claimed is:
 1. A microstructured composite material withcontrolled deformation comprising: a matrix material, the matrixmaterial being a variable elastic modulus material selected from thegroup consisting of shape memory polymer, phase changing metal, wax,ice, electrorheological fluid, magnetorheological fluid,electrostrictive material, magnetostrictive material, ferromagneticmagnetostrictive material, magnetorheological elastomer,electrorheological elastomer, and liquid crystal elastomer; at leastthree reinforcement layers embedded in the matrix material, each of theat least three reinforcement layers comprising a plurality of flatcoplanar platelets arranged along a plane of the matrix material, the atleast three reinforcement layers comprising a first reinforcement layercomprising a plurality of flat coplanar first platelets arranged along afirst plane of the matrix material; a plurality of flat connectingmembers having a flat surface extending in a thickness directioncrossing the first plane and interconnecting the plurality of flatcoplanar first platelets along the first plane of the matrix material,each of the flat connecting members connecting a corresponding two ofthe flat coplanar first platelets and having a greater height in thethickness direction than that of the corresponding two of the flatcoplanar first platelets; and means for mechanically coupling stressbetween adjacent layers of the at least three reinforcement layers tomitigate out-of-plane deformation of the matrix material while allowingdeformation of the matrix material along the first plane.
 2. Themicrostructured composite material with controlled deformation of claim1, further comprising: a reinforcement member on at least one of the atleast three reinforcement layers, wherein the reinforcement member hasan elongated shape and extends in a direction substantially parallel tothe at least one of the at least three reinforcement layers.
 3. Themicrostructured composite material with controlled deformation of claim1, wherein the plurality of flat coplanar first platelets are arrangedin a cellular pattern.
 4. The microstructured composite material withcontrolled deformation of claim 1, wherein the plurality of flatcoplanar first platelets are arranged in a lattice pattern.
 5. Themicrostructured composite material with controlled deformation of claim1, wherein each of the plurality of flat connecting members has a flatstraight section.
 6. A microstructured composite material withcontrolled deformation comprising: a matrix material; at least tworeinforcement layers embedded in and connected through their thicknessby the matrix material, each of the at least two reinforcement layerscomprising at least two platelets arranged along a plane of the matrixmaterial, the at least two reinforcement layers comprising a firstreinforcement layer and a second reinforcement layer adjacent to thefirst reinforcement layer, the first reinforcement layer comprising atleast two first platelets arranged along a first plane of the matrixmaterial, the second reinforcement layer comprising at least two secondplatelets arranged along a second plane of the matrix material; a firstconnecting member extending along a first path in the first planebetween and connecting to a corresponding two of the at least two firstplatelets, and further extending from the first path in a directioncrossing the first plane above and below the corresponding two of the atleast two first platelets; and a second connecting member extendingalong a second path in the second plane between and connecting to acorresponding two of the at least two second platelets, and furtherextending from the second path in a direction crossing the second planeabove and below the corresponding two of the at least two secondplatelets, wherein the second connecting member crosses and interlockswith the first connecting member to mitigate translation between thefirst connecting member and the second connecting member while allowingrotation between the first connecting member along the first plane andthe second connecting member along the second plane.
 7. Themicrostructured composite material with controlled deformation of claim6, wherein the matrix material is a variable elastic modulus material.8. The microstructured composite material with controlled deformation ofclaim 7, wherein the matrix material is from the group consisting ofshape memory polymer, phase changing metal, wax, ice, electrorheologicalfluid, magnetorheological fluid, electrostrictive material,magnetostrictive material, ferromagnetic magnetostrictive material,magnetorheological elastomer, electrorheological elastomer, and liquidcrystal elastomer.